1. Field of the Invention
The present invention relates to a thermal pen tip and the preparation thereof.
2. Description of the Prior Art
Recently, a thermal recording system using a thermal pen tip has been studied and practically used in various fields in view of high functional characteristics of the apparatus and low maintenance. The thermal pen tip comprises a resistant layer for heating and it is heated by feeding current to the resistant layer for heating. On the other hand, the heat is transferred to a thermal sensitive paper which is slidably brought into contact with the thermal pen tip. One of various coloring reactions such as a reduction is resulted on the thermal sensitive paper whereby a recording of figures and symbols is resulted by the coloring.
The conventional thermal pen tip usually has a structure shown in FIG. 1 wherein the thermal pen tip (1) comprises a prismatic tip shaped insulating ceramic as an insulating substrate (2) and a heat reserve layer (3) made of glaze glass in uniform thickness is formed on all of the main rectangular surfaces of the insulating substrate (2) so as to reserve a desired heat energy resulted by feeding the current through the resistance layer (4) for heating.
The resistance layer (4) is formed at the central part on the upper surface of the heat reserve layer (3) and a pair of a conductive layers (51), (53) are extended to the heat reserve layer (3) and the side surface of the substrate (2) at the end of the resistance layer (4). As the most upper layer, a protective layer (6) is formed to be substantially the same as the area of the main surface of the substrate (2) thereby preventing the wearing caused by abrasion of the thermal sensitive paper with which the upper surface is slidably brought into contact. Thus, a width of the colored line formed on the thermal sensitive paper by the thermal pen tip is depending upon an area for slidably contacting the thermal pen tip with the thermal sensitive paper.
The width of the coloring line is decided by the area of the main surface of the insulating substrate in the case of the abovementioned conventional thermal pen tip. In view of said fact and the technology for cutting the insulating ceramic for forming tips and the strength required for the tip, it is difficult to give a width and a length of the main surface of the insulating substrate less than 300 .mu.m. Therefore, it is advantageously difficult to write lines having a width of less than 300 .mu.m by the conventional thermal pen tip.
The inventors have proposed to use a thermal pen tip having the structure shown in FIG. 2 so as to overcome said problems.
In FIG. 2, the thermal pen tip (1) comprises a square prismatic tip type insulating ceramic substrate (2) and a heat reserve layer (3) formed on the main surface of the substrate (2). In said structure, the upper surface of the heat reserve layer (3) forms a convex surface having a desired curvature to at least one side of the main surface of the substrate (2) and the heat reserve layer (3) has a semispherical or semicylindrical curved configuration. The resistant layer for heating (4), a pair of the conductive layers (51), (53) and the protective layer (6) are formed on the heat reserve layer (3) are the same as those of the former embodiment.
The thermal pen tip which has been previously proposed, has a semispherical or semicylindrical curved convex configuration as the contact surface which slidably contacts the thermal sensitive paper with the thermal pen tip whereby the area for slidably contacting with the thermal sensitive paper can be remarkably smaller than that of the conventional thermal pen tip and the line recorded can be thinner with high contrast than that of the conventional thermal pen tip. However, the inventors have studied further and have found that such proposed thermal pen tip has the following disadvantages.
Firstly, the slidably contact surface of the thermal sensitive paper is curved, whereby the slidably contact area is varied depending upon fluctuation of the pen pressure or change of the curvature of the curved surface caused by fluctuation of the condition of the preparation for each product. Therefore, the accuracy and evenness of the width of the colored line are not satisfactory.
Secondly, it is difficult to decide the condition of the preparation for providing a desired curvature of the curved surface so as to give a desired width of the colored line because the slidably contact surface of the thermal sensitive paper is curved. Thus, it is difficult to prepare plural kinds of the thermal pen tips having different slidable contact surfaces by one machine.
Thirdly, although the slidable contact area can be minimized, the normal pushing pressure is too high whereby the wearing of the protective layer contacting with the slidable contact surface is too much and the width of the colored line is disadvantageously varied for a long operation.